Allow script command to load from classpath and file resources

[ezeprimo]

Fixes #1014

script --file only accepted absolute filesystem paths. It now resolves the value with DefaultResourceLoader, so classpath:... and file:... references work. An existing filesystem path keeps the previous behavior (backward compatible, no new options).

Summary:

• Script resolves --file/-f as a Spring resource, file-first fallback
• FileInputProvider gained a Reader overload reusing the line-continuation/comment logic

File	Change
spring-shell-core/.../command/Script.java	Resource resolution with file-first fallback
spring-shell-core/.../FileInputProvider.java	New Reader constructor
spring-shell-core/.../command/ScriptTests.java	New: classpath, absolute path, file-URL, missing-file
spring-shell-core/.../FileInputProviderTests.java	New Reader test

Test plan:

• mvn -pl spring-shell-core -am -Dtest=ScriptTests,FileInputProviderTests test: 6/6 green (JDK 17)
• mvn -pl spring-shell-core -am test: 222/222 green

[czpilar]

You should use your own name as an author, not mine... ;-)

[ezeprimo]

Author adjusted.

[fmbenhassine]

Thank you for the contribution! LGTM, I will merge it. However, I will add a follow-up commit to restrict resource resolution to classpath: and file: prefixes only (as the original OP in #1014 requested). The reasoning is that even though script only ever executes commands already registered in the app, the resource loading step itself (as it is currently implemented in this PR) can be pointed at other URL schemes (like http: or https:), which would make the application issue a network call to a location supplied via the --file option. Restricting to classpath:/file: closes that (security gap) off while resolving #1014 as requested.

[fmbenhassine]

Rebased, squashed and merged as 07a0e66. Refined in 7fb993b.

Thank you for your contribution!