PR description:
This PR attempts to contribute to "Remove gArgs" (#21005).
Main changes:
`GetDataDir()` function is moved to `ArgsManager.GetDataDirPath()`. `GetBlocksDir()` function is moved to `ArgsManager.GetBlocksDirPath()`.
This is a backport of core#21244 [1/8]
https://github.com/bitcoin/bitcoin/pull/21244/commits/70cdf679f8e665dbdc3301873a0267fe9faa72cd