feat: make Microsoft Entra ID tenant configurable

[paskal]

Summary

• Add MicrosoftTenant field to provider.Params and AddMicrosoftProvider method to auth.Service
• Allows single-tenant Entra ID (Azure AD) applications to specify their tenant instead of the hardcoded common endpoint
• Defaults to "common" when no tenant is specified, preserving full backwards compatibility
• Changes applied to both v1 and v2

Contributes to umputun/remark42#1998

[coveralls]

Pull Request Test Coverage Report for Build 21887694868

Details

• 19 of 19 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 84.247%

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Totals
Change from base Build 21151982006:	0.6%
Covered Lines:	2706
Relevant Lines:	3212

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💛 - Coveralls

[umputun]

clean, minimal change - does what it says. CI green, 100% coverage on changed lines, v1/v2 consistent.

two things worth considering:

1. tenant validation - the value goes directly into microsoft.AzureADEndpoint(tenant) which concatenates it into URLs like https://login.microsoftonline.com/{tenant}/oauth2/v2.0/authorize. since it's server-side config the risk is low, but basic validation (reject /, .., ?, #, whitespace) would be cheap insurance. up to you whether it's worth it.

2. AddMicrosoftProvider + UserAttributes - the new method creates empty UserAttributes. if someone needs both custom tenant and user attributes, they'd have to construct provider.Params manually. not a blocker, just noting the gap.

otherwise LGTM

[umputun]

LGTM, thx for adding the validation