fix: add linearizable parameter in FetchClusterRequest

[iGxnon]

Please briefly answer these questions:

proto changes: xline-kv/curp-proto#14

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

Fix linearizable read in FetchClusterRequest

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov]

Codecov Report

Attention: 31 lines in your changes are missing coverage. Please review.

Comparison is base (2cbb0fd) 54.13% compared to head (70e0452) 54.12%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   54.13%   54.12%   -0.02%     
==========================================
  Files         101      101              
  Lines       17851    17856       +5     
  Branches    17851    17856       +5     
==========================================
  Hits         9664     9664              
- Misses       7610     7615       +5     
  Partials      577      577              

Files	Coverage Δ
xline/src/server/cluster_server.rs	0.00% <0.00%> (ø)
xline/src/server/maintenance.rs	33.33% <0.00%> (ø)
curp/src/server/raw_curp/mod.rs	68.00% <0.00%> (-0.07%)	⬇️
curp/src/server/curp_node.rs	15.30% <0.00%> (-0.06%)	⬇️
curp/src/client.rs	12.80% <0.00%> (-0.29%)	⬇️

... and 1 file with indirect coverage changes

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[Phoenix500526]

LGTM