Rule request: `AvoidSecureStringDisclosure`

As for AvoidUsingConvertToSecureStringWithPlainText it should be avoided to retrieve a PlainText password from a SecureString as it might leave memory trials (or even logging trails).

$Password = $SecureString | ConvertFrom-SecureString -AsPlainText

Besides, the action that follows and requires the plaintext password is likely vulnerable from a security aspect.
This will also include the common used statements as:
(see: https://stackoverflow.com/a/28353003/1701026)

$SecurePassword = ConvertTo-SecureString $PlainPassword -AsPlainText -Force
$BSTR = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($SecurePassword)
$Password = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR)
[Runtime.InteropServices.Marshal]::ZeroFreeBSTR($BSTR)

and (see: https://stackoverflow.com/a/40166959/1701026):

$Password = (New-Object PSCredential 0, $SecurePassword).GetNetworkCredential().Password

The general approach of dealing with credentials is to avoid them and instead rely on other means to authenticate, such as certificates or Windows authentication.

Proposed technical implementation details (optional)

Create rules to check for the above mentioned unsafe statements.

Start by locating the existing PSScriptAnalyzer rule implementations and tests, then compare their handling of ConvertFrom-SecureString, SecureStringToBSTR, and PSCredential.GetNetworkCredential(). The work is done when unsafe SecureString-to-plaintext patterns are reliably reported and corresponding analyzer tests cover the examples in the issue.