feat: support hcv namespace

[bzp2010]

Description

HashiCorp Cloud and Enterprise editions support multi-tenancy based on the namespace concept. By using a specific HTTP Header, we can get the key from a specific namespace.

This makes sense, for HashiCorp Cloud it looks like namespaces are turned on by default and its default namespace is admin, which will not work when the client (APISIX) does not specify a namespace.

This PR adds namespace support on HCV (HashiCorp Vault).

Addition

In addition to this, I would suggest to rename the vault module in APISIX to hcv, based on the fact that vault itself is a neutral term that stands for some kind of component used to store keys. HashiCorp Vault is the name of that software, so it should be written hcv and not vault.

I would suggest renaming it, and in the meantime, we could add some sort of aliasing mechanism to continue to support users who use vault until APISIX 4.0, when we can remove support for the old name.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[bzp2010]

I noticed that a lint error has appeared in another document, it has nothing to do with the current PR, should I fix it here?

[bzp2010]

It looks like the code changes are fine, but the documentation changes still need to be checked by one of the main documentation maintainers:

1. Confirm that the placement of the description is appropriate. (I didn't find any tables to document the configurations supported by HCV)
2. where other lint errors in the documentation should be fixed

[moonming]

It looks like the code changes are fine, but the documentation changes still need to be checked by one of the main documentation maintainers:

1. Confirm that the placement of the description is appropriate. (I didn't find any tables to document the configurations supported by HCV)
2. where other lint errors in the documentation should be fixed

@kayx23 please take a look, thx

[kayx23]

@bzp2010

1. For the apache/apisix doc, the "table" you are looking for is in the Admin API reference.
2. What lint errors? I don't see any CI checks failing.

[bzp2010]

Since this PR modifies the Chinese admin-api.md documentation, it also fixes the documentation lint error.