Ungenericify Deepgram struct

[algesten]

Proposed changes

The Deepgram<K> struct is generic over the API key K. The signature is:

pub struct Deepgram<K>
where
    K: AsRef<str>,

I propose K is cloned and stored internally in a String.

Context

There is no obvious benefit to the API user to be generic over an API key. I can see two reasons to do this, and neither seem to hold up.

1. Save space. However it's 40 bytes as a string (and looks like a hexadecimal number, so I assume it can be stored as a number too?). It seems to be a very small gain.
2. Allow user to control storage of the API key. In a security paranoid environment maybe it would be possible to hold the key in a "secure enclave" or similar tech. Though AsRef would defeat this, since a simple operation is supposed to release the key. If I don't want the Deepgram struct to remain in memory with the key, I can simply drop the struct. If the API want to ensure the key is zeroed out from memory, the API can impl Drop for Deepgram and zero the memory where the key was stored.

Possible Implementation

 fn new(key: impl AsRef<str>) -> Self {
    let api_key = key.as_ref().to_owned();
    Self { api_key }
 }

[lukeocodes]

hey thanks for this @algesten we'll try and review it asap