Add information about the input into the property binding process to the configprops endpoint

[wilkinsona]

When trying to determine why a property on a @ConfigurationProperties bean has a particular value, it would be useful to know the input into the binding process that resulted in the value. For example, this would make it possible to see if any placeholders are used in the original input.

Relates to #8293.

[philwebb]

See also #14880

[mbhave]

This also closes this issue.