feat(header): Implement find method on headers

[nikhars]

Add the ability in the Headers struct of types to be able to find a key and if found return the corresponding value.

[Swatinem]

Suggested change

	assert_eq!(headers.get("key1"), Some(b"value1".to_vec()).as_deref());
	assert_eq!(headers.get("key2"), Some(b"value2".to_vec()).as_deref());
	assert_eq!(headers.get("key1"), Some(b"value1"));
	assert_eq!(headers.get("key2"), Some(b"value2"));

[nikhars]

The suggested change gives the following error
error[E0308]: mismatched types
--> rust-arroyo/src/backends/kafka/types.rs:162:41
|
162 | assert_eq!(headers.get("key1"), Some(b"value1"));
| ^^^^^^^^^^^^^^^ expected Option<&[u8]>, found Option<&[u8; 6]>
|
= note: expected enum Option<&[u8]>
found enum Option<&[u8; 6]>

error[E0308]: mismatched types
--> rust-arroyo/src/backends/kafka/types.rs:163:41
|
163 | assert_eq!(headers.get("key2"), Some(b"value2"));
| ^^^^^^^^^^^^^^^ expected Option<&[u8]>, found Option<&[u8; 6]>
|
= note: expected enum Option<&[u8]>
found enum Option<&[u8; 6]>

Didn't know rust would be so restrictive. Made a change to map the value and return its ref.