v6: Backups API

[bevzzz]

Merge #339 to reduce diff.

This PR adds Backup API with support for all features from v1.36 (cancel restore, incremental backups) and previous releases.

Key changes:

• backup/client.go - new namespaced client, i.e. the public API
• internal/api/backup.go - implementation of the supported REST requests
• backup/wait.go - AwaitSuccess and AwaitStatus utils
• backup/client_test.go, backup/wait_test.go, internal/api/endpoint_test.go -- unit tests

Miscellaneous:

• internal/testkit/context.go - adds "ticking context", which expires after Done has been called a certain number of times. Useful for mocking context timeouts at fine granularity.
• ErrorIs in internal/testkit/error.go is similar to the ExpectError but allows asserting specific a concrete error type.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   115   594   611	View in Orca
Warning	SAST	1   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

🛡️ The following IaC misconfigurations have been detected

	NAME	FILE
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	JSON '$ref' alongside other properties	...st/schema.check.json	View in code
	Empty Array	.../rest/schema.v3.yaml	View in code
...	...	...	...

🛡️ The following SAST misconfigurations have been detected

	NAME	FILE
	Leaky Use of time.After in for-select Statements	backup/wait.go	View in code

Note: The scan should have failed if no policies were configured in warn-only mode.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Secrets	0   0   0   0	View in Orca

[tsmith023]

question: should this branch be targeting v6-collections rather than v6 since it includes the collections/ directory?