feat: allow new tags in schema again

[mgattozzi]

This commit restores the old behavior we had where new tags can be added to a schema. To do this we made tags nullable and brings us in line with our other products. These changes were made in this PR:

influxdata/influxdb3_core#41.

Changes to accomplish this new behavior were:

• Queries now do not return an empty string for null tags instead they are returned as null, or in many formats not at all.
• References to v1 for parsing and validating lines were removed as we only have one path for doing so these days shared amongst all the write_lp endpoints.
• We fixed failing tests that expected us to not be able to have new tags or depended on that functionality indirectly
• Tests had their snapshot files updated to reflect that tags are nullable by default
• Behavior for making a schema and checking whether a column can be null were updated in a separate repo and integrated here
• The series_key is updated whenever we get a new tag added to the schema
• New tests were added to show that you can add a new tag and that the series key is updated as part of that

With the above changes we can now allow tags to be added again by users like they would expect, especially with v1 and v2 apis and Telegraf plugins.

Closes #25932
Closes #26065

[mgattozzi]

Corresponding Enterprise PR -> https://github.com/influxdata/influxdb_pro/pull/534

Pretty much the same thing as this, but with updated Enterprise tests. I'll sync these changes into Enterprise once this is approved and get that PR moving along.

[pauldix]

Great stuff, thanks for working through all the back and forth on it