chore: encode jwt key and cert digest with base64 in logs

[iTrooz]

Really small PR, just thought jwt_key and cert_digest were taking too much space in logs, and could easily be reduced by encoding them in base64. I also removed cert_pem, since it showed as cert_pem: [...1058 bytes...]

Before:

[2026-05-17T00:18:09Z TRACE sccache_heartbeat] Req 0: heartbeat_server: HeartbeatServerHttpRequest { jwt_key: [25, 54, 10, 14, 31, 226, 31, 242, 7, 245, 8, 116, 18, 112, 251, 219, 200, 173, 157, 132, 108, 116, 68, 57, 228, 213, 30, 81, 76, 182, 10, 72], num_cpus: 12, server_nonce: ServerNonce(11202272517116080065), cert_digest: [102, 111, 244, 156, 4, 40, 162, 175, 129, 80, 176, 154, 64, 5, 36, 208, 126, 204, 96, 10, 0, 192, 151, 126, 156, 4, 50, 72, 83, 155, 2, 220], cert_pem: [...1050 bytes...] }

After:

[2026-05-17T00:14:58Z TRACE sccache_heartbeat] Req 1: heartbeat_server: HeartbeatServerHttpRequest { jwt_key: "5Gg6mVLJNGeMU1LvKI7KLOewxFlz1GI_Ef6NAieeHYI", num_cpus: 12, server_nonce: ServerNonce(15425886545403093633), cert_digest: "pc89Scum-6zejZQIOEY-UTPXbnV2DWHQ4exPoddCDqg" }

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.34%. Comparing base (c0db872) to head (00d5eb0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dist/http.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2712   +/-   ##
=======================================
  Coverage   74.34%   74.34%           
=======================================
  Files          70       70           
  Lines       39373    39367    -6     
=======================================
- Hits        29271    29268    -3     
+ Misses      10102    10099    -3     

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