feat(oauth): add support for X/Twitter v2 provider

[issuedat]

Adds support for X (formerly Twitter) v2 as an external OAuth provider.

• Introduces an oauth_states table to persist the code_verifiers as X mandates the use of PKCE
• Uses SHA256 challenge for PKCE
• Updates the GetOAuthToken signature to accept a code_verifier as the second parameter
• Uses the existing cleanup middleware to delete states
• Adds the provider as x rather than x_v2 as twitter is already used in old OAuth 1.0a provider to better align with the rebrand
• The state is a UUIDv4

NOTE: today the flow_states table is overloaded, containing states, auth codes, provider tokens...the goal is to decouple that table eventually and the oauth_states table is the first step towards that.

[coveralls]

Pull Request Test Coverage Report for Build 19871055030

Details

• 254 of 317 (80.13%) changed or added relevant lines in 31 files are covered.
• 4 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.2%) to 68.435%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/api/provider/twitter.go	3	4	75.0%
internal/api/provider/slack_oidc.go	2	5	40.0%
internal/models/oauth_client_state.go	17	21	80.95%
internal/api/provider/apple.go	2	7	28.57%
internal/api/provider/slack.go	0	5	0.0%
internal/api/provider/spotify.go	0	5	0.0%
internal/models/errors.go	0	5	0.0%
internal/api/external.go	33	39	84.62%
internal/api/provider/x.go	80	86	93.02%
internal/api/provider/linkedin_oidc.go	0	7	0.0%

Files with Coverage Reduction	New Missed Lines	%
internal/api/provider/linkedin_oidc.go	1	0.0%
internal/api/provider/slack.go	1	0.0%
internal/api/provider/spotify.go	1	0.0%
internal/api/provider/vercel_marketplace.go	1	0.0%

Totals
Change from base Build 19864408688:	0.2%
Covered Lines:	14641
Relevant Lines:	21394

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💛 - Coveralls

[cstockton]

If possible I would like to remove context.Background while we are touching these code paths.

[cstockton]

🚀