RFR: 8354724: BufferedReader readAllLines and readString methods

[Brian Burkhalter]

Implement the requested methods and add a test thereof.

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Commit messages:
 - 8354724: BufferedReader readAllLines and readString methods

Changes: https://git.openjdk.org/jdk/pull/24728/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24728&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354724
  Stats: 176 lines in 2 files changed: 175 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24728.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24728/head:pull/24728

PR: https://git.openjdk.org/jdk/pull/24728