Do not pin `serde` crate in the `hyperswitch_masking` crate

### Description

When the `hyperswitch_masking` crate was introduced (copied over) in #8, it pinned the `serde` crate to a specific version (`1.0.228` in #8, see [this](https://github.com/juspay/framework-libs-rs/blob/0461795faed668c6333d5f3b49e12f4dad5df146/crates/hyperswitch_masking/Cargo.toml#L32)), since the `FlatMapSerializer` type is available from a "private" module. Moreover, the private module has `serde`'s patch version included in the fully qualified path (`__private228` in the below code snippet), which would break if dependents of the `hyperswitch_masking` crate use a different version of the `serde` crate in their code:

https://github.com/juspay/framework-libs-rs/blob/0461795faed668c6333d5f3b49e12f4dad5df146/crates/hyperswitch_masking/src/serde.rs#L151

Identify a long-term fix for this, and do not pin `serde` to a specific version after this is fixed.