Add ECDSA key support for key-pair authentication

[jesusmgg]

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-3017609: Support ECDSA keys for key-pair authentication #2743

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am modifying authorization mechanisms
   • I am adding new credentials
   • I am modifying OCSP code
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

   Adds ECDSA key support (ES256, ES384, ES512) for key-pair authentication.

   Snowflake server supports ECDSA keys, but the Python connector only accepted RSA. This change:

   • Accepts EllipticCurvePrivateKey in addition to RSAPrivateKey
   • Detects curve type (P-256, P-384, P-521) and selects the appropriate JWT algorithm
   • Raises a clear error for unsupported curves

4. (Optional) PR for stored-proc connector:

   N/A

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[jesusmgg]

I have read the CLA Document and I hereby sign the CLA

[jesusmgg]

recheck

[jesusmgg]

Heads up: the CLA bot seems broken - returning "Repository was archived so is read-only" when saving signatures.

[jesusmgg]

recheck

[jesusmgg]

Bumping this, the CLA check is still failing. The bot confirmed the signature but the check stays red. @sfc-gh-dszmolka any chance someone could look into this?

[sfc-gh-turbaszek]

@jesusmgg please add appropriate change log entry to DESCRIPTION.md

[sfc-gh-turbaszek]

Renaming ALGORITHM to ALG_RS256 removes a public class constant, which is a breaking change for any downstream code referencing AuthByKeyPair.ALGORITHM. Consider preserving a backward-compatible alias:

ALGORITHM = ALG_RS256  # deprecated, kept for backward compatibility