fastp: add v0.23.4

[alecbcs]

Add fastp v0.23.4.

Test Plan:
Built successfully using gcc@10.4.0 on Debian 11.

[spackbot-app]

Hi @alecbcs! I noticed that the following package(s) don't yet have maintainers:

• fastp

Are you interested in adopting any of these package(s)? If so, simply add the following to the package class:

    maintainers("alecbcs")

If not, could you contact the developers of this package and see if they are interested? You can quickly see who has worked on a package with spack blame:

$ spack blame fastp

Thank you for your help! Please don't add maintainers without their consent.

You don't have to be a Spack expert or package developer in order to be a "maintainer," it just gives us a list of users willing to review PRs or debug issues relating to this package. A package can have multiple maintainers; just add a list of GitHub handles of anyone who wants to volunteer.

[tldahlgren]

Confirmed the version sha256.