Add feature flags to spacetimedb-sdk to control TLS

[bfops]

Description of Changes

Fixes #2878.

For some reason, even though I didn't change the default features, the Cargo.lock still got a bunch of new stuff. ChatGPT thinks it's because of something something unification with the dependencies that reqwest uses.

API and ABI breaking changes

None. Default features have not changed.

Expected complexity level and risk

1

Testing

• No more openssl dependency when using the features!
  Default:

$ cargo tree -pspacetimedb-sdk -i openssl -e=no-dev
openssl v0.10.74
└── native-tls v0.2.14
    ├── tokio-native-tls v0.3.1
    │   └── tokio-tungstenite v0.27.0
    │       └── spacetimedb-sdk v1.8.0 (/home/lead/work/clockwork-localhd/SpacetimeDBPrivate/public/sdks/rust)
    ├── tokio-tungstenite v0.27.0 (*)
    └── tungstenite v0.27.0
        └── tokio-tungstenite v0.27.0 (*)

With the feature flag:

$ cargo tree -pspacetimedb-sdk -i openssl -e=no-dev --features tls-rustls-native --no-default-features
warning: nothing to print.

To find dependencies that require specific target platforms, try to use option `--target all` first, and then narrow your search scope accordingly.