feat: support arbitrary delimiters in acc-role profile names

[markjreed]

Description

If the cred_profile configuration parameter has the form "accXrole" for any character X, it is treated as "acc-role" with X as the delimiter between account and role in the generated profile name.

Related Issue

#474

Motivation and Context

See the issue.

How Has This Been Tested?

Added two new tests, one with a colon and resolve_alias true, the other with a slash and resolve_alias false.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  Technically will break if anyone is using a literal profile name that fits the pattern, e.g. 'acc@role'

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.