chore: introduce ConnectorSplit protobuf message

[shanicky]

What's changed and what's your intention?

PLEASE DO NOT LEAVE THIS EMPTY !!!

as title

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #3274 (19d5da3) into main (53a49d8) will increase coverage by 0.00%.
The diff coverage is 10.52%.

@@           Coverage Diff           @@
##             main    #3274   +/-   ##
=======================================
  Coverage   73.23%   73.24%           
=======================================
  Files         747      747           
  Lines      101789   101783    -6     
=======================================
+ Hits        74545    74546    +1     
+ Misses      27244    27237    -7     

Flag	Coverage Δ
rust	73.24% <10.52%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/connector/src/base.rs	40.00% <ø> (ø)
src/connector/src/datagen/source/reader.rs	86.66% <ø> (ø)
src/connector/src/lib.rs	100.00% <ø> (ø)
src/connector/src/nexmark/source/reader.rs	92.70% <ø> (ø)
src/connector/src/state.rs	83.33% <0.00%> (+1.14%)	⬆️
src/meta/src/barrier/command.rs	75.59% <0.00%> (ø)
src/meta/src/stream/source_manager.rs	26.34% <0.00%> (+0.11%)	⬆️
src/source/src/connector_source.rs	0.00% <ø> (ø)
src/stream/src/executor/mod.rs	51.59% <0.00%> (+1.93%)	⬆️
src/connector/src/macros.rs	8.16% <4.54%> (-2.65%)	⬇️
... and 1 more

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[tabVersion]

LGTM