fix: count string by RuneCountInString in StringLenBetween()

[tky2240]

Related Issue

Fixes:

• [Bug]: Fixed-response in ALB listener rule fails with multibyte characters exceeding 342 characters  terraform-provider-aws#37802
• ELBv2 FixedResponse with message_body (<= 1024 chars && > 1024bytes) cannot be registered  terraform-provider-aws#22949

Description

Use RuneCountInString in StringLenBetween function. This fix resolves an issue that multibyte characters were not be counted correctly.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

None

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