[pkg/ottl] Added ottl functions for hashing strings

[rnishtala-sumo]

Description:
Added 3 ottl functions for hashing strings

• FNV
• SHA1
• SHA256

Link to tracking Issue: #22725

Testing:
Added the following unit tests

• FNV
• SHA1
• SHA256

Documentation:

• https://github.com/rnishtala-sumo/opentelemetry-collector-contrib/tree/ottl_hash_functions/pkg/ottl/ottlfuncs#converters

[TylerHelmuth]

I didn't look at the details of the functions yet but the general structure of this PR is correct. Please add entries for each function in the ottlfuncs README.

[evan-bradley]

Thanks for adding these. Overall looks good to me, just one question.

[kentquirk]

I know I'm swooping in late, but could we please mention in the documentation that the SHA1 and SHA256 functions are not recommended to be used except where compatibility requires it? They're slow, insecure by modern standards, and really shouldn't be used by new applications.

[rnishtala-sumo]

maybe a note like below?

Note: SHA1 is not a recommended hash function by the National Institute of Standards and Technology (NIST) due to the availability of more secure alternatives like SHA3.

[kentquirk]

That's OK, but we don't include SHA3 as an available function. Maybe instead:

Note: According to the National Institute of Standards and Technology (NIST), SHA1 is no longer a recommended hash function. It should be avoided except when required for compatibility. New uses should prefer FNV whenever possible.