feat: Add db.system indicators to redis, sqlalchemy, and django

[AbhiPrasad]

ref getsentry/team-webplatform-meta#60

dependent on getsentry/develop#917

Add this information to span data field as db.system, matching OpenTelemetry's well known conventions.

For example, db.system of postgresql would indicate that it's a postgres database.

[AbhiPrasad]

I realized this strategy doesn't work if you have multiple connections open, but not sure how common that is.

[antonpirker]

In Django it is common to have one connection to a read only replica of the database and one you use to change the data, so probably on bigger installations quite common.

[AbhiPrasad]

Splitting this up in favour of 4 PRs:

• Adding db.system constant: Add db.system constant #2037
• Add redis feat(redis): Add db.system to redis db spans #2038
• Add sqlalchemy feat: Add db.system to sqlalchemy spans #2039
• Add django feat(django): Add db.system #2040