feat: introduce Headers data type for HTTP

[algebraic-dev]

This PR introduces the Headers data type, that provides a good and convenient abstraction for parsing, querying, and encoding HTTP/1.1 headers.

This contains the same code as #10478, divided into separate pieces to facilitate easier review.

The pieces of this feature are:

• Core data structures: feat: add core HTTP data types #12126
• Headers: feat: introduce Headers data type for HTTP #12127
• URI: feat: introduce URI data type for HTTP #12128
• Body: feat: introduce Body type class and some Body types for HTTP #12144
• H1: feat: introduce HTTP/1.1 protocol state machine #12146
• Server: feat: introduce HTTP/1.1 server #12151
• Client:

[ghost]

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[leanprover-bot]

Reference manual CI status:

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[Rob23oba]

I'm a bit confused by the isEmpty decidable instance but otherwise some small improvements and comments