Import BBS archive with migrate-repo command

[synthead]

Closes #528!

This PR updates the placeholder migrate-repo command for bbs2gh with real logic to ingest an archive URL and kick off a migration with Octoshift.

A few notes about this PR:

• There are some parameters that pass strings like "not-used" to satisfy required parameters in GraphQL and Octoshift. When https://github.com/github/octoshift/issues/5350 is shipped, these "filler" strings can be removed.
• CreateBbsMigrationSource was added to GithubApi. This is consumed by the bbs2gh MigrateRepoCommand to kick off migrations over GraphQL.
• bbs2gh.csproj was added as a reference to OctoshiftCLI.Tests.csproj. This is necessary to test classes in the OctoshiftCLI.BbsToGithub.Commands namespace.
• I opted to use a MigrateRepoCommandArgs class in bbs2gh's MigrateRepoCommand because I felt that it was cleaner and easier to read.
• Some unused ADO logic was pulled from bbs2gh's EnvironmentVariableProvider.
• As for test coverage, I opted to find parity in the coverage for the GEI tests that is relevant to bbs2gh. I'm happy to add more coverage if desired 👍

Tasks

• Did you write/update appropriate tests
• Release notes updated (if appropriate)
• Appropriate logging output
• Issue linked
• Docs updated (or issue created) (added bullet point to Document bbs2gh migrations #586)

[github-actions]

Integration Test Results

4 tests   4 ✔️  12m 15s ⏱️
1 suites  0 💤
1 files    0 ❌

Results for commit 553391c.

♻️ This comment has been updated with latest results.

[github-actions]

Unit Test Results

481 tests   481 ✔️  25s ⏱️
    1 suites      0 💤
    1 files        0 ❌

Results for commit 553391c.

♻️ This comment has been updated with latest results.

[dylan-smith]

#586 is the tracking issue for docs, that will come later when we're getting ready for private beta.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
gei	86%	86%	343	✔
ado2gh	88%	84%	498	✔
bbs2gh	44%	24%	51	❌
Octoshift	92%	78%	674	✔
Summary	87% (4373 / 5000)	80% (933 / 1162)	1566	✔