Store masp-params in base dir

[ruuda]

On version f1afdffd5e43ad4bb448db7bf5bc1e23464350f7, which is the required version for using namada-cq-2.a6ebeb093671093b21, anoma ledger tries to access ~/.masp-params, and there doesn’t appear to be a way to change this path.

When running as a system user, which is standard practice for daemons, the user may not have a home directory, and so the application crashes.

Relevant part of strace

statx(AT_FDCWD, "/nonexistent/.masp-params/masp-spend.params", AT_STATX_SYNC_AS_STAT, STATX_ALL, 0x7ffc38b9b9d0) = -1 ENOENT (No such file or directory)
write(1, "MASP parameters not present, dow"..., 44MASP parameters not present, downloading...
) = 44
mkdir("/nonexistent/.masp-params", 0777) = -1 ENOENT (No such file or directory)
mkdir("/nonexistent", 0777)             = -1 EACCES (Permission denied)
statx(AT_FDCWD, "/nonexistent", AT_STATX_SYNC_AS_STAT, STATX_ALL, 0x7ffc38b9b770) = -1 ENOENT (No such file or directory)

The user's home directory is /nonexistent, which indeed does not exist.

Instead of storing things at a hard-coded location under ~/.masp-params, I think it would make sense to put this data in the base dir. If that is not desirable for some reason, the path should at least be configurable. I looked into the code and it’s a third-party crate that hard-codes the path, but it looks like it is part of Anoma and ultimately the issue surfaces in anoma, so I am opening an issue here.

Also for the default path, consider respecting the XDG Base Directory Specification instead of creating yet another entry directly in the user's home directory.

[cwgoes]

@mariari can you take this quickly as part of the MASP work (low priority)?

[mariari]

It seems to be related to the work in #100 and thus should be done after #685 is completed