use builder style API for process spawning in event loop

[Guest0x0]

Use builder style API for creation of process spawning job, so that we can more easily add more options in the future. Also reserve an is_orphan flag for the process spawning API for potential future use.

Since #518 would require changing the API interface, I think we may also adapt the change here to Wasm as well for cleaner FFI shape? cc @peter-jerry-ye

[coveralls]

Coverage Report for CI Build 941

Coverage decreased (-0.07%) to 76.986%

Details

• Coverage decreased (-0.07%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (4 of 5 lines covered, 80.0%).
• 2 coverage regressions across 2 files.

Uncovered Changes

File	Changed	Covered	%
src/internal/event_loop/thread_pool.mbt	5	4	80.0%

Coverage Regressions

2 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
src/internal/os_string/os_string.mbt	1	50.0%
src/tls/transport.mbt	1	83.33%

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Coverage Stats

Relevant Lines:	4858
Covered Lines:	3740
Line Coverage:	76.99%
Coverage Strength:	11830.27 hits per line

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💛 - Coveralls