User functions for building SCRAM secrets

Hi,

We currently do not provide any SQL functions for generating SCRAM 
secrets, whereas we have this support for other passwords types 
(plaintext and md5 via `md5(password || username)`). If a user wants to 
build a SCRAM secret via SQL, they have to implement our SCRAM hashing 
funcs on their own.

Having a set of SCRAM secret building functions would help in a few areas:

1. Ensuring we have a SQL-equivalent of CREATE/ALTER ROLE ... PASSWORD 
where we can compute a pre-hashed password.

2. Keeping a history file of user-stored passwords or checking against a 
common-password dictionary.

3. Allowing users to build SQL-functions that can precompute SCRAM 
secrets on a local server before sending it to a remote server.

Attached is a (draft) patch that adds a function called 
"scram_build_secret_sha256" that can take 3 arguments:

* password (text) - a plaintext password
* salt (text) - a base64 encoded salt
* iterations (int) - the number of iterations to hash the plaintext 
password.

There are three variations of the function:

1. password only -- this defers to the PG defaults for SCRAM
2. password + salt -- this is useful for the password history / 
dictionary case to allow for a predictable way to check a password.
3. password + salt + iterations -- this allows the user to modify the 
number of iterations to hash a password.

The design of the patch primarily delegates to the existing SCRAM secret 
building code and provides a few wrapper functions around it that 
evaluate user input.

There are a few open items on this patch, i.e.:

1. Location of the functions. I put them in
src/backend/utils/adt/cryptohashfuncs.c as I wasn't sure where it would 
make sense to have them (and they could easily go into their own file).

2. I noticed a common set of base64 function calls that could possibly 
be refactored into one; I left a TODO comment around that.

3. More tests

4. Docs -- if it seems like we're OK with including these functions, 
I'll write these up.

Please let me know if you have any questions. I'll add a CF entry for this.

Thanks,

Jonathan

P.S. I used this as a forcing function to get the meson build system set 
up and thus far I quite like it!