Integrate "Generic hashing function"

Closes #134 (closed)

Based on the !142 (closed) MR:

• Add CI tests
• Add documentation for the hashing functions
• Change documentation of the pseudonymization section
• Store the seed in the secret table
• Store the hash algorithm in the secret table
• PG10 and below only support md5 / use pgcrypto
• Add a demo
• Add a note about security definer functions in CONTRIBUTING
• Add a note about performances with dynamic masking