feat: detect if org is a devhub at auth time

[cristiand391]

What does this PR do?

Updates AuthInfo class to always setisDevHub field based on a server-check.

Before:
It was up to consumers to set the field based on a server-check or anything else like passing a flag in a plugin

After:
sfdx-core will do a server-side check and set the field to either true or false.

QA steps:

1. Clone plugin-auth
2. yarn link @salesforce/core into ⬆️
3. test all auth commands both with/without --setdefaultdevhubusername
4. Even without passing --setdefaultdevhubusername , you should see the isDevHub field in the authfile.

What issues does this PR fix or reference?

@W-10999290@

[salesforce-cla]

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[peternhale]

QA Notes:

Using testing dev hub, ran the auth commands below, logging out between attempts.
Success for the test is finding property isDevHub set to true in the resulting auth config file for the user.
Also confirmed that previous version of core DID NOT set isDevHub property in auth files prior to start of testing.

• auth:jwt:grant
• auth:web:login
• auth:sfdxurl:store
• auth:access:token:store

All of the commands properly set the isDevHiub and also set alias and set org to default devhub username when requested.

Tested non-devhub org with web login, yielding isDevHub property value of false.