remote_cache: Fix redis connstr parsing

[mblaschke]

What this PR does / why we need it:
Fixes wrong splitting string after the first =, allows usage of base64 passwords like Azure Redis is using.
Instead of split the string into multiple pieces only split it into max of 2 pieces.

Which issue(s) this PR fixes:
Fixes #18199

Special notes for your reviewer:

[CLAassistant]

All committers have signed the CLA.

[papagian]

Thank you for finding this and contributing your modifications!
I have proposed some additional changes before merging.

[papagian]

Anyway, instead of returning a different error I would proposed to censor the rawKeyValue.
In that case you could do something like this:

	for _, rawKeyValue := range keyValueCSV {
		keyValueTuple := strings.SplitN(rawKeyValue, "=", 2)
		censored := []byte(rawKeyValue)
		if len(keyValueTuple) != 2 {
			if strings.HasPrefix(rawKeyValue, "password") {
				for i := len("password"); i < len(rawKeyValue); i++ {
					censored[i] = '*'
				}
			}
			return nil, fmt.Errorf("incorrect redis connection string format detected for '%v', format is key=value,key=value", string(censored))
		}

[mblaschke]

Instead of leaking any information (eg. length of the password) what about the current solution? I think nobody is interested in the correct length so the error should be enough.

[papagian]

That is even better. Just a small improvement.