Allow usage of custom .cargo/config.toml options

[Suya1671]

Is your feature request related to a problem? Please describe.
I am attempting to use this with a project that uses simdjson, which requires me to edit the config.toml file with extra options (specifically setting target-cpu=native)

Describe the solution you'd like
The ability to specify extra config.toml options in fleet.toml

Describe alternatives you've considered
None

Additional context
Error from simdjson

error[E0308]: mismatched types
   --> /home/tntman/.cargo/registry/src/github.zerozr99.workers.dev-1ecc6299db9ec823/simd-json-0.4.15/src/lib.rs:222:86
    |
222 | fn please_compile_with_a_simd_compatible_cpu_setting_read_the_simdjsonrs_readme() -> ! {}
    |    ----------------------------------------------------------------------------      ^ expected `!`, found `()`
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
    |
    = note:   expected type `!`
            found unit type `()`

config values I need:
https://github.com/simd-lite/simd-json/blob/main/.cargo/config#L1-L2

[VarunPotti]

Will look into implementing this soon.

[haaarshsingh]

We'll take a look into this.