Use map instead of array for headers variable in InternetHeaders

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**Is your feature request related to a problem? Please describe.**
Referring to this variable, https://github.com/eclipse-ee4j/mail/blob/master/mail/src/main/java/jakarta/mail/internet/InternetHeaders.java#L277, I believe there is merit in making the headers variable a Map. Key would be header name and value would be a list of Strings (to account for same header occuring multiple times). This makes header lookups via getHeader() method more performant by avoiding having to loop through the list everytime (https://github.com/eclipse-ee4j/mail/blob/master/mail/src/main/java/jakarta/mail/internet/InternetHeaders.java#L465)

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
Use map instead of list.

**Describe alternatives you've considered**

**Additional context**
If you are comfortable with it, I can implement and raise a pull request for the same.

Start in mail/src/main/java/jakarta/mail/internet/InternetHeaders.java around the headers declaration near line 277 and the getHeader() implementation near line 465. Trace all accesses to the headers variable and verify that repeated header names remain supported; the work is done when lookups use the new map representation without changing observable header behavior.

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