Add MVC retry backoff support

[goutamadwant]

Summary

• Add backoff support to MVC RetryConfig with firstBackoff, maxBackoff, factor, and basedOnPreviousValue
• Apply configured backoff in both MVC retry implementations: GatewayRetryFilterFunctions and FrameworkRetryFilterFunctions
• Document YAML and Java DSL usage and add regression coverage for both retry paths plus nested config binding

Fixes #4141

Testing

• ./mvnw -pl spring-cloud-gateway-server-webmvc -Dtest=RetryFilterFunctionBackoffTests,RetryFilterFunctionSelectionLogicTests,RetryFilterFunctionNoSpringRetrySelectionTests test
• ./mvnw -pl spring-cloud-gateway-server-webmvc -DskipTests verify

[ryanjbaxter]

@spencergibb I wonder if we should just move to using Framework retry. However just glancing at the code it looks like it might involve breaking changes due to some interfaces we implement.

[spencergibb]

I think spring-retry is around until the next major

[ryanjbaxter]

basedOnPreviousValue does not appear to be used in the MVC implementation...I am not sure it makes sense fir MVC

[ryanjbaxter]

It probably makes sense to do some additional validation here

Objects.requireNonNull(this.firstBackoff, "firstBackoff must be present");
    Assert.isTrue(this.firstBackoff.toMillis() >= 1, "firstBackoff must be >= 1ms");
    Assert.isTrue(this.factor > 1, "factor must be greater than 1");
    if (this.maxBackoff != null) {
        Assert.isTrue(this.maxBackoff.compareTo(this.firstBackoff) > 0,
                "maxBackoff must be greater than firstBackoff");
    }