feat: add hand-written Prometheus remote write request parser (#1628)

## Rationale

A hand-written remote write request parser to replace prost, with the
primary goal of achieving zero-allocation parsing.

## Detailed Changes

- The new `remote_write` directory contains the core implementation of
the hand-written parser.

- Add benchmarks.

- Add a .proto file in `pb_types`.

- Fix CI errors.

## Test Plan

- Unit tests can be found at the end of the `pb_reader.rs`.

- A comprehensive equivalence test is available in
`equivalence_test.rs`, which validates the correctness of the
hand-written parser by comparing its output with that of the prost
auto-generated parser.